GMAT and TOEFL tests - preparation necessary
Where these tests are applicable, study for them.  Although there is a review course for the GMAT, it is not always necessary, and sometimes does more harm than good since they are taught at the pace of the slowest learner in the class. 

Buy one or two good books to help you and study on your own at your own pace, at the time of day that suits you best.

The TOEFL (test of English as a foreign language) is definitely to be studied for by those students from abroad wishing to gain acceptance to an English-speaking institution.